The field of art to which this invention pertains is the catalytic hydroprocessing of hydrocarbons to useful hydrocarbon products. More particularly, the invention relates to catalytic hydroprocessing and the recovery of useful hydrocarbon products such as diesel.
Petroleum refiners often produce desirable products such as turbine fuel, diesel fuel, middle distillates, and gasoline boiling hydrocarbons among others by hydroprocessing a hydrocarbon feedstock derived from crude oil or heavy fractions thereof. Hydroprocessing can include, for example, hydrocracking, hydrotreating, hydroisomerization, hydrodesulfurization and the like. Feedstocks subjected to hydroprocessing can be vacuum gas oils, heavy gas oils, and other hydrocarbon streams recovered from crude oil by distillation. For example, a typical heavy gas oil comprises a substantial portion of hydrocarbon components boiling above about 371° C. (700° F.) and usually at least about 50 percent by weight boiling above 371° C. (700° F.), and a typical vacuum gas oil normally has a boiling point range between about 315° C. (600° F.) and about 565° C. (1050° F.).
Hydroprocessing is a process that uses a hydrogen-containing gas with suitable catalyst(s) for a particular application. In many instances, hydroprocessing is generally accomplished by contacting the selected feedstock in a reaction vessel or zone with the suitable catalyst under conditions of elevated temperature and pressure in the presence of hydrogen.
Hydrocracking is a type of hydroprocessing that is generally accomplished by contacting in a hydrocracking reaction vessel or zones a gas oil or other feedstock to be treated with a suitable hydrocracking catalyst under conditions of elevated temperature and pressure in the presence of hydrogen so as to yield a product containing a distribution of hydrocarbon products desired by the refiner. The operating conditions and the hydrocracking catalysts within a hydrocracking reactor influence the yield of the hydrocracked products.
Hydroprocessing and most notably, hydrocracking, produce problematic compounds known as heavy polynuclear aromatics (HPNA's). Polynuclear aromatics (PNA) are compounds that have two or more aromatic rings. HPNA's typically refer to compounds with six or more aromatic rings and often refer to compounds with eleven or more aromatic rings. HPNA's can build up in a hydroprocessing recycle loop and accelerate catalyst deactivation.
U.S. Pat. No. 6,379,535 taught a catalytic hydrocracking process which uses a split-shell fractionator to recover lower boiling hydrocarbon product streams, a liquid recycle stream and a drag stream containing a high concentration of heavy polynuclear aromatic (HPNA) compounds. The split shell fractionator receives just one external product feed stream. A portion of the fractionator bottoms from one side of a split shell fractionator is fed to the other side of the split shell in which steam is used to strip lighter materials from the bottoms and leave a concentrated HPNA bottoms stream. U.S. Pat. No. 6,379,532 similarly taught a similar split shell arrangement to concentrate HPNA bottoms. However, two external product feed streams are fed to the split shell fractionator, a lighter product feed stream and a heavier product feed stream are both external feed streams fed to opposite sides of the split shell fractionator. The heavier feed stream derived from a separator upstream of the split shell fractionator is stripped to concentrate HPNA's in a liquid stream.
The amount of stripping steam required to strip lighter materials from the HPNA's in the split shell fractionation column is substantial. Combined with the substantial stripping steam requirements on the other side of the split shell fractionation column for stripping lighter materials from the main feed to the column, the stripping steam requirement is quite large. The steam requirements have implications for the size of the product fractionation column itself and for the overhead condenser system. Additionally, it is desirable for the fractionation column to provide specified product cuts efficiently.